bcm5974(4): Properly assign MT-slot on Apple Magic Trackpad

Assign multi-touch slot number based on internal evdev MT state and
reported tracking ID of contact rather than on sequentional number of
contact in report.

Sponsored by: Serenity Cyber Security
Fixes: ef8397c28e98 ("add Magic Trackpad 2 (USB only) support")
MFC after: 1 month